MIPS: loongson: use IS_ENABLED()

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3336/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>